This invention relates to reactive, 100 percent solids, segmented, phase-separating, polyether polyurethane prepolymers. More particularly, it relates to the preparation of such prepolymers, the novel properties of the prepolymers, novel processing made possible by the prepolymers, the elastomers resulting from the prepolymers and the products made from the elastomers.
It has heretofore been known to prepare polyurethane polymers having elatomeric properties by three different chemical routes, characterized principally on the basis of processing considerations. (See for instance, Hepburn, C., Polyurethane Elastomers, Applied Science Publishing Ltd., 1982 and Saunders & Frisch, Polyurethanes Chemistry and Technology, Part II. Technology, Robert E. Krieger Publishing Company, 1983). These three groupings of elastomers are distinguished by processing as a liquid, millable rubber or thermoplastic.
The building blocks generally employed in polyurethane technology, the isocyanate species, the polyol and chain extender are well known in the art. Depending on the equivalency ratios employed using these building blocks, various characteristics are achieved that dictate both the processing techniques to be employed to achieve the final desired product, and the properties of the final product. FIG. 1 illustrates, in a triangular coordinate plot, the percentage equivalents of isocyanate, polyol and chain extender utilized in a general way in prior polyurethane elastomer technology.
Region A of FIG. 1 delineates the equivalency ratios commonly employed to obtain a chain extended, segmented, high molecular weight thermoplastic polyurethane exhibiting elastomeric properties. The theoretical maximum molecular weight in this reaction (one-shot or prepolymer) of difunctional reagents is achieved when the equivalency ratio of diisocyanate to active hydrogen species (polyol and chain extender) is one. Thus, thermoplastic polyurethanes are substantialy centered around the region of 50 percent equivalents of isocyanate.
The thermoplastic route to a solid polyurethane elastomer can be subdivided into two classifications: those completely soluble in certain solvents and containing no chemical crosslinks before and after processing, and those materials possessing no initial crosslinks, but which form a lightly crosslinked structure after a heated post-cure.
The former class is the more predominantly encountered and is most commonly made by the reaction of essentially equivalents of isocyanate and active hydrogen functionality or a slight excess of the active hydrogen component. Products from this class have the drawback in that they are inherently sensitive to particular solvents and will swell extensively in some solvents and dissolve in others. This limits their applications in some areas of application such as coatings, adhesives and sealants.
In the second class of thermoplastic polyurethanes, the synthesis is similar, however, a slight excess of isocyanate is employed to generate a final polymer having a small amount of unreacted isocyanate groups. These isocyanate groups are then available for crosslinking the final polymer through allophonate and biuret formation. The crosslink density is low by this method and the final thermoplastic polyurethane polymer must be given a heated post-cure to "activate" these residual "dormant" isocyanate groups.
The processing of either class of thermoplastic polyurethanes may proceed by melt processing techniques and, in the former class, by solution techniques. Melt processing, such as injection molding, extrusion and calendering, generally requires fairly sophisticated equipment and high temperatures frequently approaching degradation temperatures of the thermoplastic polyurethane itself. As a rule, these products have a high molecular weight and high melting point. Although low melting thermoplastic polyurethanes are known, the strength properties of such polymers, including their tensile strength, percent elongation and tear propagation resistance, are generally poor. Solution systems usually require very polar solvents such as tetrahydrofuran, dimethylformamide, dimethylsulfphoxide, M-Pyrol, which necessitate such concerns as environmental factors, the higher cost for solvents, and energy.
The millable rubber route to a solid polyurethane elastomer falls into Region B of FIG. 1. To allow conventional rubber processing techniques to be applied to the polymers, of this class, the polymers therein are distinguished by their being chain terminated during synthesis by employing an excess of either the chain extender or the polyol, resulting in a storage stable, soluble polymer of lower molecular weight than the thermoplastic polyurethanes. To achieve adequate final physical properties, the prepolymer is generally either further chain extended or crosslinked by employing additional isocyanate, or, where appropriate, cured by sulphur or peroxide.
Liquid processing leading to polyurethanes having elastomeric properties may be further subdivided into the prepolymers existing in Region C of FIG. 1, or those referred to commonly as "casting" systems.
Cast polyurethane elastomers are made by a process wherein the reactants are mixed in the liquid state (prepolymer or one-shot route), the reacting mixture is then fabricated into its final form by an appropriate technique such as casting or molding, and the reaction leading to chain extension/crosslinking continues. Complete cure is typically achieved by a hot air post-cure for three to twenty-four hours at 100.degree. C. In general, after the chain extender has been added and mixed with the prepolymer (or all three components mixed in the one-shot technique), the reaction of these species limit the subsequent pot life to several minutes (see for instance, the improvements provided in U.S. Pat. No. 3,471,445). Because this method involves the mixing of two or more liquids, which are all generally of low molecular weight, it is found that initial physical properties of the system are poor until the curing proceeds to some degree. The equivalency ratios of isocyanate:polyol:chain extender employed in cast polyurethane elastomers places most of these systems close to the region characteristic of thermoplastic polyurethanes, generally with a tendency to a slight excess in isocyanate, although, in principle, these systems may employ quite varied equivalency ratios.
Alternatively to the chain extension of the prepolymer, as discussed previously, the prepolymer, Region C of FIG. 1, has been utilized directly. Curing is normally achieved by chain extension of the prepolymer through the reaction of the isocyanate groups with water and crosslinking by allophonate and biuret formation. It is in this use that a distinguishing feature is observed between the polyester prepolymers and the polyether prepolymers.
The physical form of these prepolymers ranges generally from a viscous liquid to a waxy, low melting solid, dependent usually upon whether a polyether or polyester polyol has been employed, respectively. In general, polyether prepolymer systems do not exhibit any of their final physical properties until substantially along in the cure cycle. Many polyester prepolymer systems, due to the inherent tendency of the polyester segment to crystallize, exhibit many of their final physical properties early in the cure cycle. This processing characteristic of polyether prepolymers limits many of their industrial applications, wherein some integrity, "green strength", low flow or similar characteristics are required.
Another classification of polyurethanes yielding cured polymers having elastomeric properties are those provided by a "blocked" isocyanate, "one-package" method, in which a polyol is employed as a mixture of a polyisocyanate (block isocyanate) masked with a blocking agent. The blocked isocyanate method presents disadvantages in that it requires relatively high temperatures for curing to eliminate the blocking agent. When the blocking agent remains partially in the resulting cured polymer, the agent will adversely affect the physical properties of the elastomer and cause environmental pollution in association with the scattering of the agent. These disadvantages permit limited use of the resulting resin.
Although limited, there are some prior polyurethane elastomers that would fall into the equivalency ratios of isocyanate, polyol and chain extender represented by Region D of FIG. 1. Those elastomers from Region D are generally characterized by having an excess of the isocyanate species. The preponderance of the elastomers from this region are fabricated according to the casting techniques described above. Representative of this class are cast Adiprene/MOCA systems described in Saunders and Frisch, Polyurethanes Chemistry & Technology, Part II Technology, Robert E. Krieger Publishing Company, 1983, pps 345, 350. As mentioned, a limitation to the casting technique has been the limited pot life after all reagents are admixed.
Driven by EPA restrictions, the high cost of solvents and the energy to drive them off, as well as the increased awareness of the toxicity of many solvents, the past decade has seen a trend to higher solids systems for adhesives, coatings and so forth. The difficulty in 100 percent solids systems has been in achieving certain physical characteristics such as green strength with processing characteristics that do not necessitate the frequently encountered situation of a need for highly specialized equipment.
U.S. Pat. No. 2,917,486 discloses that an intermediate from the equivalents of Region D may be stored for subsequent later processing by the addition of a stabilizer. The stabilizers prevent premature gelling during storage or processing. Stabilization, however, must later be overcome by the addition of additional isocyanate. It is also recognized in the art to employ acyl halides such as p-nitrobenzoyl chloride, in catalytic amounts, in prepolymers to stabilize against crosslinking during storage and to facilitate processing.
U.S. Pat. No. 3,049,513 provided for ordered, polyester isocyanate terminated components yielding compositions for coatings having superior physical properties than obtained by other, then-available, isocyanate components. Either two component systems, usually in solution, where the polyfunctional isocyanate species was employed as one of the components, or a one component, moisture-cured solution system were provided.
U.S. Pat. No. 4,273,911 discloses low melt temperature processible thermoplastic polyurethanes having acceptable final physical properties by addition of two melting point lowering diol chain extenders and one strength enhancing diol chain extender. Therein, an acceptable compromise between melt behavior and final physical properties is achieved.
Numerous attempts have been made to prepare useful polyurethane elastomers, which are both melt processable and have acceptable final physical properties, from a polyol of polyoxypropylene. See, for instance, U.S. Pat. Nos. 3,915,937 and 4,239,879. Systems having the chemical resistance to hydrolysis of polyether urethanes, and which possess both the economics of low temperature melt processing and lower raw material cost of the poly(oxypropylene)glycol over the commonly employed poly(oxytetramethylene)glycol and acceptable physical characteristics, would be strongly desired.
Likewise, numerous attempts have been made to prepare useful polyurethanes, which are melt processable and have acceptable physical properties, from a polyol of polyoxyethylene. See, for instance, U.S. Pat. No. 3,164,565 or U.S. Pat. No. 3,901,852, where, in the latter reference, successful systems were prepared within the narrow window of both a substantially balanced weight ratio of hard segment to soft segment and an isocyanate to active hydrogen equivalency ratio of approximately 1:1. This is limiting however, particularly when it is desired to produce products wherein the polyoxyethylene content influences other characteristics of desired products, such as hydrophilicity.
U.S. Pat. No. 4,367,327 discloses a film of polyoxyethylene polyurethane to be utilized as a solution cast textile coating providing breathability and waterproofness. A compromise is made between the polyoxyethylene content for breathability and the elastomeric physical properties required for product performance.
Historically it has been difficult to obtain required physical characteristics such as ultimate tensile strength, ultimate elongation, modulus of elasticity, tear strength, and abrasion resistant characteristics in a highly hydrophilic polymeric coating which is useful and commercially attractive. This becomes even more difficult if the economics require melt processability. Specifically, films of highly hydrophilic polymers have tended to be weak and either easily torn or damaged by abrasion and/or flex, especially when swollen with water. Accordingly, there is a current need to produce melt processable systems having increased hydrophilicity without the heretofore concomitant deterioration in physical properties.
It is well known in the art that polyurethane polymers exhibit excellent elastomeric properties, particularly those of the (AB).sub.n segmented block copolymer type, where the polyol soft segment (A) alternates with the polyurethane hard segment (B). It is widely accepted that the unique properties of these copolymers are directly related to the two-phase microstructure which exists when the hard and soft segments phase-separate, the hard segments forming domains which act as a reinforcing filler and pseudo-crosslink the polymeric network.
Accordingly, it is an object of the present invention to overcome the difficulties alluded to hereinabove and provide storage stable, moderate temperature melt processible, one component, 100 percent solid, reactive polyurethane prepolymers which, when cured, yield elastomers possessing the excellent physical properties typical of the (AB).sub.n type segmented urethane copolymers.